Cisco 400-007 Exam With Confidence Using Practice Dumps

PortFast is a feature that allows access ports to enter the STP forwarding state immediately, without waiting for the usual listening and learning states. This accelerates the connection of end devices like PCs or servers.

A key design benefit is that PortFastdoes not trigger topology changeswhen devices are plugged in or disconnected. This prevents unnecessary STP recalculations and protects core stability.

Correct Statement (A):STP does not consider link transitions on PortFast-enabled ports as topology changes, reducing control-plane overhead.

Other options misrepresent the function of PortFast:

B:PortFast does not disable STP; it bypasses specific STP states.

C/D/E/F:These describe other features like Loop Guard or UDLD, not PortFast.

==========

—

Let’s calculate total 1-year cost for each option using the provided table:

DWDM over dark fiber:

CAPEX = $250,000

OPEX (1 year) = $100,000

Installation fee = $30,000

Total = $250,000 + $100,000 + $30,000 = $380,000

CWDM over dark fiber:

CAPEX = $150,000

OPEX (1 year) = $100,000

Installation fee = $25,000

Total = $150,000 + $100,000 + $25,000 = $275,000

MPLS wires only:

CAPEX = $50,000

OPEX (1 year) = $80,000

Installation fee = $5,000

Total = $50,000 + $80,000 + $5,000 = $135,000

BUT: MPLS does not allow customer-controlled QoS policies as stated in the requirement (cannot run its own varying QoS profiles without SP interaction). Therefore disqualified.

Metro Ethernet:

CAPEX = $45,000

OPEX (1 year) = $100,000

Installation fee = $5,000

Total = $45,000 + $100,000 + $5,000 = $150,000

BUT: Metro Ethernet is also typically a service provider-managed solution where customer-controlled QoS is usually limited depending on SP capabilities. May not fully meet the requirement.

—

The key technical requirement:

“Be able to run its own varying QoS profiles without service provider interaction.”

This clearly favors dark fiber solutions (CWDM and DWDM), as these give full Layer 1 control to the customer, allowing complete freedom in managing QoS, traffic engineering, and prioritization internally.

Among the two dark fiber options, CWDM over dark fiber is the most cost-effective choice for one year at $275,000.

—

Therefore, final correct answer:

B (MST — Multiple Spanning Tree): Bundles multiple VLANs into fewer STP instances, minimizing the scope of TCN propagation, which helps reduce MAC table flushing and unicast flooding.

Why other options are incorrect:

A: PVRSTP still processes TCNs per VLAN, increasing flooding.

C: Legacy STP behaves similarly but is not optimized for multiple VLANs.

D: PVSTP+ generates TCNs per VLAN like PVRSTP.

—